Although carbenes have been suggested as being toxic metabolites of several halogenated compounds, there has been no direct proof of their formation. We have now developed a method for unequivocally identifying carbenes as metabolites. The procedure is based on employing 2,3-dimethylbutene as a carbene trapping agent. As an example of the method, carbon tetrachloride was incubated with rat liver microsomes in the presence of 2.3-dimethylbutene. Gas chromatography mass spectral analysis of the reaction mixture revealed that 1,1-dichloro-2,2,3,3-tetramethylcyclopropane was a metabolite. This product was formed by the rapid reaction of 2,3-dimethylbutene with dichloromethyl carbene. Cytochrome P-450 appeared to catalyze the reaction because it required NADPH as a cofactor and was inhibited by carbon monoxide. The same trapping procedure may be used to determine whether other halogenated compounds, such as halothane, DDT and chloramphenicol are metabolized to carbene metabolites and to evaluate the importance of carbenes as toxic metabolites.